usage_stats: fix datetime.utcnow() deprecation warning

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/usage_stats.py

@@ -16,7 +16,7 @@ import argparse
 import gzip
 import re
 import sys
-from datetime import datetime
+from datetime import datetime, timezone
 from pathlib import Path
 from urllib.parse import urlparse
 
@@ -266,7 +266,7 @@ def make_figure(df: pd.DataFrame, output: Path) -> None:
     span_days    = (df["ts"].max() - df["ts"].min()).days + 1
     fig.suptitle(
         f"bincio — {total_logins} logins over {span_days} days "
-        f"· generated {datetime.utcnow().strftime('%Y-%m-%d')}",
+        f"· generated {datetime.now(tz=timezone.utc).strftime('%Y-%m-%d')}",
         color=FG, fontsize=12, y=0.97,
     )